Typical rare earth permanent magnets include permanent magnet of the SmCo.sub.5 system and a permanent magnet of the Sm.sub.2 Co.sub.17 system. These samarium cobalt magnets are produced using the following procedures: An ingot composed of samarium and cobalt is made by mixing samarium and cobalt and then melting the mixture in a vaccum or an inactive atmosphere. After the ingot has been crushed into fine powder, the powder is molded in a magnetic field and a green body is obtained. A permanent magnet is made by sintering the green body and then heat treating the sintered body.
As mentioned above, the samarium cobalt magnet is provided with magnetic anisotropy by being molded in a magnetic field. The magnetic properties of the magnet are improved substantially by providing such magnetic anisotropy. Anisotropic resin-bonded permanent magnets can be obtained by mixing crushed powder from a sintered anistropic samarium cobalt magnet with resin and molding the powder in a magnetic field, either by injecting it into a molding die or by compressing it in a molding die.
In this way, a resin-bonded samarium cobalt magnet can be produced by first making a sintered magnetically anisotropic magnet and then by crushing and then mixing it with resin.
As compared with the samarium cobalt magnet, a rare earth magnet of a new type, that is, a neodymium-iron-boron magnet, has been proposed. Japan Patent Laid-Open Nos. Showa 59-46008 and Showa 59-64733 have proposed that, in the same way as in a samarium cobalt sintered magnet, an ingot of the neodymium-iron-boron alloy be prepared, crushed into fine powder, and molded in a magnetic field to obtain the green body. By sintering the green body and heat-treating the sintered body, a sintered permanent magnet is prepared. This method is called the powder metallurgy method.
Apart from the abovementioned powder metallurgy method, a different manufacturing method of the Nd-Fe-B system permanent magnet has been proposed in certain Japanese Patent Laid-Opens as follows:
______________________________________ (Japanese Patent Laid-Open) (Based on U.S. Pat. Application) ______________________________________ No. 59-64739 No. 414,936 (Sept. 3, 1982) No. 508,266 (June 24, 1983) No. 60-9852 No. 508,266 (June 24, 1983) No. 544,728 (Oct. 26, 1983) No. 60-100402 No. 520,170 (Aug. 4, 1983) ______________________________________
According to these publications, after neodymium, iron and boron have been mixed and melted, molten metal is rapidly quenched using such technology as spinning. The Nd.sub.2 Fe.sub.14 B alloy is crystallized by heat-treating the resulting flakes of the noncrystalline alloy. Patent Laid-Open No. 60-100402 describes technology as to furnish the isotropic magnetic alloy with magnetic anisotropy by forming a green body by a hot press procedure and thereafter causing plastic streaming in a part of the green body under high temperature and high pressure. This NdFeB magnet has the following problems:
Firstly, although the abovementioned powder metallurgy process provides a magnet with magnetic anisotropy and the obtainable magnetic property is as high as 35-45 MG Oe, its Curie point is substantially low, its crystal grain size is also large, and its thermal stability is inferior compared to samarium cobalt magnets. Accordingly, these NdFeB magnets have not been widely used for motors, etc. operating in a high temperature environment.
By contrast, although mixing a powder made from the rapidly-quenched flakes with resin could theoretically make compression molding comparatively easy, the obtainable magnetic property of the bond magnets so obtained is low because of the magnetic isotropy of the powder. For example, the magnetic property obtainable by injection molding of the isotropic powder would be (BH)max=3-5 MGOe and the one obtainable by compressing molding would be (BH)max=8-10 MGOe. In addition, the magnetic property would depend on the strength of the magnetizing magnetic field. In order to obtain (BH)max=8 MGOe, the strength of the magnetizing magnetic field of about 50 KO3 would be required and it would be difficult to use this magnet in applications requiring magnetization after it has been assembled.
The hot pressing of the rapidly-quenched powder would improve the weather-proof property as the result of the density increase which makes the magnet free of voids, but since it has isotropy, it has the same problems as in the case of a permanent magnet made by directly mixing the rapidly-quenched powder with resin. Although the obtainable (BH)max would be increased because of the increase in density such that about 12 MGOe is obtainable, it is still impossible to magnetize it after assembled due to the large applied field required.
By causing plastic streaming of the rapidly-quenched powder after a hot press, it would be possible to furnish the magnet with magnetic anisotropy in the same way as in the case by the powder metalurgy process and obtain a (BH)max of 35-40 MGOe. However, it would be difficult to make a ring type magnet (for example, a magnet of 30 mm outside diameter.times.25 mm inside diameter.times.20 mm thickness) because the use of an upsetting process would be required to furnish the magnet with the required magnet anisotropy and dimensional control, especially of relatively small articles, is exceedingly difficult with such a process.
As described at pages 670-672 of the Applied Physics Letters 48 (10), Mar. 1986, it is possible to furnish a magnet with magnetic anisotropy by crushing a melt-cast ingot into powder having a grain 0.5-2 .mu.m and then making a bond magnet by solidifying the crushed powder with wax. However, on account of the fineness of the powder, its flamability makes handling it in air virtually impossible. In addition, since the squareness ration of the demagnetization curve of the powder is comparatively low, the magnet cannot provide a high magnetic property.
In an attempt to obtain a bond magnet with magnetic anisotropy, a sintered magnet with magnet anisotropy made by the powder metallurgy process was crushed, the crushed particles were mixed with resin and the magnet body was molded in a DC magnetic field. However, the magnetic properties in characteristic of the present invention were unobtainable.